1. Technical Field
The present invention relates in general to radial bearings and, in particular, to a system, method and apparatus for scale resistant radial bearing designs for electrical submersible pump components and assemblies.
2. Description of the Related Art
In many downhole pumping systems, such as rotating equipment like electrical submersible pumps (ESP), gas separators and intakes, the problem of scale build up is observed in the clearances of radial bearings. Scale may include any kind of surface deposit that might tend to develop due to environmental exposure during operation of the equipment. One problem is that the formation of scale impedes the axial movement or stroke of the shaft (i.e., the rotating assembly stack) relative to the stationary support housing. This problem can become critical even when the amount of scale build up is very thin (e.g. on the order of 0.001 inches or more).
Referring to FIG. 1, a conventional radial bearing 11 typically comprise stacked sleeves 13 (one shown) on the shaft 15 where all of the sleeves are formed at the same diameter and engage the bushing 17. For example, some radial bearing bushings and sleeves have a total diameter difference or clearance of about 0.003 to 0.015 inches between their inner and outer diameters, respectively. Scale deposits develop in the clearance on the outer surface of the sleeve that protrudes axially beyond the bushing inner surface. Upon any shaft axial stroke, the scale build up is forced into the tight clearance 19 between the bushing 17 and sleeve 13. As the scale build up is drawn into the clearance, a tremendous frictional drag is introduced in the radial bearing.
A compounding issue for radial bearings is the presence of a chamfer 21 on the face edges of the bushings 17 and sleeves 13. As the shaft 15 is axially stroked, the chamfers 21 on the leading edges act like a funnel or cam to force more scale into the bearing clearance 19. The additional friction due to these issues can cause numerous common failure modes. For example, the bearing and/or sleeve can overheat, the bearing can fail due to loss of lubrication and overheating, and the sleeve can seize inside the bushing.
In addition, the scale can limit the life or prevent reuse of the pump, gas separator or intake due to limited axial shaft stroke or seized shaft. Moreover, the pump can lock up and prevent the motor from starting, and extreme heating can cause motor failure. Furthermore, extreme frictional drag can cause shearing of the key alignment feature that is located under the sleeve, and then continued operation may result in extreme wear and weaken or destroy the shaft. Thus, an improved design that overcomes the limitations and problems associated with prior art designs would be desirable.